Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
~3~
BACKGROUND OE' THE lNVENTION:
. . . _ --
This invention relates to a process for the protection of
rolled sheets, as a rule previously galvanized, particular
ly suite~ for use in the motor-car industry. This patent,
moreover, relates to the produc-t obtained by such a process.
It is known to use electrolytic treatments based on Cr-CrOx
layers on bare (unshielded) steel. As shown by U~Ko pater.t
N 1~247,~81, U.S. patent N 3,6427587, and French ~aten-t
N 2,0039981, such electrolytic treatments are resorted to
e 5 serltially ~iith a view -to substituting, in some cases~ the~
white latten usually applied to metal con-tainers and to con
StitUtillg a pre-treatment of a bare steel side before its
~irk
~L~36~79~
hot dip galvanizing, the purpose being to prevent zinc 1ro~.
adhering to one of the sides, thus obtaining a hot dip one
side produet.
~reneh patent N 2,053,0~8 on the other hand relates to a
~ingle-stage treatment suceessive to the galvani.zing opera
tion, which does not eause ~ multi-laJer plating, but a
Cr~CrOx mixture, wherein CrOx is predominant. Also the maxi
mum deposit amount is e~ual to 0.650 g/m2 9 whieh 9 aceording
to experienee, is sub-optimal for corrosion protection.
Moreover, the treatment aCcoraing to said Freneh patent
N 2,053,038 has inconveniences as ~ar as its indu.strial im
plementation is eoneerned, and its plating is un~alaneed in
the direetion of high CrOx contents~ hich leads to a erit~
eal dissolutiGn of the plating in either aeid baths (phos-
phating) or signifieantly alkaline baths (pre-painting wash
ings)~ Consequently there are two unaceeptable effee-ts, par
ticularly in the motor-ear industry; a great variance in
eorro3ion-resistanee tests~ and eontamination of phosphating
blths.
Based on British pa~ent 1.331.844, there is also
l~nown a one- or two-stage process for the multi-layer treat
~iert of ~al~l~nizcd or zinc-alloy plated product~, according
to which a Cr-CrOx plating is applied in the zinc layer,
the said plating having a protective function of the
galvani~ed product.
The process according to the above said British
patent essentially derives from an experiment in plating
galvani~ed steel wires and cables. However, the possibility
o extending it to flat products is also mentioned.
Perhaps on account of this lack of appllcation experience
to flat rolled sheets, the process conditions specified
therein and verified industrially proved unsuited to obtain
a Cr-CrOx plating suitable for successive phosphating and
painting processes without functional and ecologic incon-
veniences.
OBJET OF THE INVENTION:
The invention relates to a process ~or the protection
of flat steel rolled sheets previously plated with zinc or
zinc containing alloys, comprising:depositing on the
surface of the previous plating a layer of chromium metal
having a thickness of at least about 0.55 g./m.2 by
electrodeposition; and then depositing on the surface of
the layer of chromium metal a layer of chromium oxide by
electrodeposition, wherein the chromium oxide layer has a
thickness of at least about 0.035 g./m. and the weight
ratio of the chromium metal to the chromium contained in
the chromium ox:Lde in the respective deposits is from
25:1 to ~
According to the present invention, and as will be
po:Lnted out in the specification of the deposition mechanism
of tlle trlvalent chromium cathodic film,a~d oE its succes-
slve ~eaction with the OH ions generated in the bath, toobtain chromium hydroxide, two basic parameters are to be
observed:
- the bath pH cannot be the one established by the
water solution of chromium anhydride (CrO3), it being
less than 1, but it must be modified by the addltion of
a base. (e.g. NaOH~. A previous rising of the bath pH
is al~o f~nda~e~t~l
/
/
/
~ 3a -
- 4
~ ~ 3~
to prevant the che~r~leal chro~late treatment which take3 pla
ce when the p~l is less th~n ~. The said chroM~-te treatment
is not applicable to car oodies, as it consirts of compound.s
o~ highly toxic hexavalent chromiurn compounds, which wou].d
con1;~millate the phosph~tation bath3 and inhl~i-t the palnt
adhesion ~)rocecs.
~ the curren-t density shall not exceed a limit value (20 .
A/d~ n ordeL to prevent the coriling ln~o being o~ higher
potenllals whlch ~ould lead ~o a discharge of meta~. ch~or~ium
instead of trivalen-t chromium.
In the industrial r-production of the conditions speciIied
in the s~id Gerrnan pa-tent application, i-t has always been
found that, because the p~ .~as highly acid, there w~s a con
temporary chromate treatrr.ent of -the ~urface.
This WtlS proven by the presence of hexavalent chrorniurn ions,
with colouLs ran~in~s fror~ golden yellow to green and blue~
as al~o descIibed in the apI:lication. The products -thus re
llzed were not considered accepttlble by the motor vehi.cle
industry
or the~3e rcasons, the fundar~lental conditions of the proce3s,
-the subject natter O.r -this inventi.on, fall wi.thin ran~es Or
vari~tion Nhich are .vider than those provided for by -the
5 -
36~79~L
above said British patent 1. 331. 844 .
The fund1mental parameters for the two-stage electrol~tic
process .vhich is the most ~uited for one of the products,
de~cribed, are:
1st STAGE ~ Depo~ition of ~,e_al chro..iu~.
- Concentration of chromium anhydride
- Pre~ence of catalyzing elements in ad~ition to
sulphate ion~
- Opti~um te~perature range
- Optimum current density current range
2nd STAGE : Deposition of oxide ~enerating trivalent chro~ r
- Concentration of chromium anhy~ride
- Presence of catalyzing ele~ents
- Current density range
~ PH modification so as to ~revent chromate treat
ment and facilitate a reaction between Cr~3 and
0~ ~
- Optimum temperature range
The above parameters are fundamental for the functional cha
racteristics of the plating, as well as from the ecolog:ical
point of view.
~23~79~
~he scope of thi~ invention con3ists in a proce~s for the
protection Or galvanized flat steel rolled sections, vihlch
by means of multilayer electrolytic pl~ting" permits an i~
proved protection ôf the said rolled sections, ~ith no ne-
~r~l:L t i ~ e 9 ~ 3 e r f' e c t 5 .Th~.s anfl ol;hel scopes O.e the i.nvention will be lila~e clear
to t~le experts in the a~t by the .~ollowing specifications
and cla:i.rrls~
D.~SC~IPTION OF ~l~HE II~VENTlON:
The process according to the invention consists in deposi-
ting by electrolytic ~ieans one or IQore layers o~ inorganic
elemen-ts or compouncls on top of the ælnc base layer~ ~ore
par-ticularly, the process is characteri.ze~ i.n -that the alec
tro].ytic pl!lting consis-ts Oe a layer o~ metal chromium and
a layer o e chromiuM o~ide, the said plating being obtalna~l.e
by means o.~ a -two- or one-stage electrolytic process.
l`he process according to the invention can be carried out
in con-tinuity at the end porti.on of a hot dip galvanizing
plan~ or of arl electrogalvanizing plant usin~ %inc or zirlc
all.c)ys, whcltever the type of the plant (horizontal cel.ls,
vertical cells, ci.rcular or radial cells, tight-sealed cells
679~
for the high recirculation speed o~ the electrolytic solu-
tion) carousel cells, and others~, or9 f`inally, in a self-
containing pllnt, indepen.dent of any other plating plant,
whe-the.r upstrearn or downstream.
It is to be unler3tood that the following sirnplifications
will be used in the speci~`ic~tion:
- the ~ord "steel" stand3 for ~lat rolled steel section3,
up to 2,500 rrl~ wide, and 10 m~n thick, cold or hcJt rol.led 9
in the forrn of rolls or sheets;
- the ~lording "zinc-based platin~" stands for steel pla-
tings made ~ith zinc or zinc alloys.
~he zinc-based pl-ating thickness according to the deIinition
i3 to be Gon~sider~d as 1 to 100 ~m on each plated side;
- the wordings "galvarlized" or "zinc-plated steel.s" stand
for steels plated with zinc or zinc all.oys, either on one
or on both sides, by rneans of whatsoever process, such as
dipp:ing into a rnolten b-lth, or an electrolytic ~rocesY, or
the ap~lication o~ pow~e:rs;
- the WOI'~ "galvrlnizing" stand3 ror any ~)rocess sui-t1ble to
.late a steel surtace ~ith zinc o.r zinc al~oy3;
- the word "multil.ayer" s-tands for two or more superim.posed
~36~
pl~ting layers, apr,~ied in a continuous cr discon-tinuous
succession, to the same or to different installations, 1.vhere
in the fir3t plating layer, in contact ;vith ~he steel, is
the zinc-based layer, whatever its Ivay o~ api,lication;
the word "transport" stands for r~otor-vehicles, rnotor-
oycle3, bicycles, indu~tri~l vehicles, ~arrning or buildin~
tractorC3~ bùse~L3, trains, 3hip.~ and boats;
- ~he ~Jord "body" stands ~or al~ tran~port parts made ~Yith
flat steel rolled sections: body~.~orks, chassis, suspensions,
wheel3, ~tructural and covering elerJents.
- "trivalent chromium" stand~ for a iO~l mix~ture essentially
constituted by Cr~3, wherein there may be present ~tates
ha.ving a valence other than that of chromium (e.g. bivalent
chro~ium);
- "chromium hydroxide" and "chromium oxide" stan~ for com-
pound~-3 essentially of trivalent chromium, wherein there n.ay
be present also valence states other than chromi.um (e.g. b:L
v~llen-t) for which the ~ormulas Cr(OH)x and CrO~ will bè used.
']1he growi.ng need to pre~erve materi~ls and ~oodcj~ i,n the
pre~ent conditions oY shortclge ~md high cost Oe raw materi.al~
and of the power required for their .extr~cti.on and trans:for
~3~
mation into durable oods, leads to consider with grreater
attention the ways and means to protect steel f`rom the main
cause that shortens its li~e: corrosion.
Trlnsport r~arlufacturing are a hug~ industry, rnaking a great
u~e o[` steel. It is theretore understandable that in the
la~-t decade rnotor vehicle manuf~cturers have paid special
attention to the problerrl of safeguarding car bodies LrorrL
corro3ion, in ~act all those parts -~hich are made from ~`la-t
steel rollel sections~
Besides improving the painting method~, attenpts were made
to find radical solutions to the steel corrosion resistance
problem by means o~ sectlons pre plated ~lit`h ~.inc-based pro
ducts, that i'3, by re~orting to those steel plating products
which take advanta~e Or the cathodic or sacrilical protect
ion from corrosion oLfered by zinc.
Unfortunately, ho~ever, the ~3.1id zinc plated ~teels~ v/hen
sublnitted to ~ complex trans~ormation into finished trar~
port coruponents (pre3sing, ~eldin~, painting), and success
ively used in he~vy environmen-t corrosion conditions have
~isclosel a number o~ sub~tarltial shortcomin~.
E'or this reason also, t~le app:Lication of steel pre-plated
- 1 o -
~3~9~L
~vith ~inc-base~ l)ro~uct3 has been less extenled than i,t
could have been expectel'on account of the s ~criLlcal. pro-
tection ~u~ranteed by thi.s type of r,roduc-tsO
Moreover, irl those cac3es ~vhere the s~id ap~lic.~tion is dcsem
ed indisperl3able the tnotor car indu.stries ~us-t a~'f:ord higher
tr~lri3for~ tlon costs thcln those which have long been provi
ded ~OI' the u3e o~ steels not previously pl~ted (pressin~
ccr~lpJj ,~reldin~ electrodes 11ear, h~rnl~ul ~/el~ling fumec3, p~lnt
ing proble~s, reduced productivi,ty, deprecia-tion o~ zinc-
cont,~in~te~ scrc1ps, etc.)
Ihe rationale of this i.nvention lS ~hat; in or~er t'o ~.ake
galvanized steel maintai,n ~ll the ~ositive proprietle;3 o~
z1nc, and 1.oose or reduce the ne~ative ones ~or its exploit
ation in motor vehicle~ ~nd similar ~pplications, the sur-
f~ce pre-plat~d with a ~inc-b.l~e pro~uct sh()uld be plated
wi-th one or more successive 1.ayers, o~ opti.mca1. ~olding,, ve].d
ing, [~ainti.ng and corro~ion resi~.3t~lnce characteri.sti.cc3.
It ~Ja~ f'ound out that the ~es-t ~y to I~ut this in l:~rlcti.ce
i~ to leposit electro].ytic-lly one or more laycr.q o~ lnorga
nic cleïnents or co~:pouni~s on to~ o~ -the ~lnc-b~s~d ~ayer.
In view of' the i'act th~-t the ~inc-base~ ~,re~-~2:Lciti.n~,r can be
done on one or both sides o~ -the flat s-tcel rolle~ secti.on,
~L~3~9~
the follo~1ing eleetrolytic ~1eposit combinations are possi-
ble:
- on one ~ide only, (previously galvanized) in the ease Ol'
"one sicle" zinc-base pre-plating;
~ on both ,g~lvanize~l c3i~es, in the ease of "two ~ide" zinc-
.1 .9 e plating;
- on one ~ide only of' the two previously galvani~ed sides
. -J.n ease of "two si~e" zinc-base plating. In this case the
electroly-tic deposit is not present on one of the two zine-
base si~es.
Ideally, the eleetrodeposition of suceessive layers on %ine~
base ~re-~ilating takes plaee on the same galvanizing line;
to the final ~ortion of it suitable installations are a~e~.
E~owevQr, eleetrocleposition is also possible outsitle the ga~
vani~ing line, in a separate installation, built for that
~eci.fio l~urpose. This invention eovers both in.c~tallation
pOS ~ .lbiliti.es.
This inventior1 re~er~ also to all sorts of electrolytie de
r~o~its of` organie layers, ho~ever built and ~Ihatever their
nurrlbt-~r; nevt3rtheless, ~,~e shall herein~ter rt3fer 1nore s~ec
i'ieally to the Yaet that t~o electro].ytic layer~ :r'o]low the
g~:1.vlni~in~ l~rocess~ a metal ehrorniurn layer ancl a trivllent
- 12 -
~L~3~79~
chromiu31 layer, ~hich, through a cherl1ical reaction Nith the
b.-lth, tran3forms itself firstly into chro~ium hy~ro~ide ~nd
th~n in-to chromiurll oxide, by dehy~ration.
The.c3e ~re inde~.1 the electrol~rtic la~ei9 ( Cr - CrOx) ~hich,
aftQr e~terl~sive exE)erimenting, have proven to be the ~it-tes-t
to solve proble~s i.n connec-tion ~vith -the production of mo-
~orve~h:i.cle bodies~
ln motor~/ehic].e },anufacturing, the optlunum ~teel ~lating ic
expected to sho.~l -the ~ollow:i.ng characteristics:
A. MOl.Dli~lG_~Er~UIRL'L/l1.~NTS
A.1 The Ir.oldability should be equal to that of the basic
steel.
A.2 The ~lating s'rlould not flake of~.
A.~ It should not contamina-te the mold.s.
A.4 If possible, is 3hould act as a lllbricant i.n the -.teel
secti.on - mol~ interactions.
A.5 Its conta!ninating ele~ant content shouli nGt be ;such as
to ~epreciate -the mold scraps.
W:~T,D.ING K~QIJI~ .qEN'rS
__ ~ _____
B.1 The platin~ should not harilE)er t~e rnechclnic ch~r-.lcterist
ics of ~.~/elding .spots~
B.2 It should no-t increa~e the ~earing ~ei)th~ nor the need
to bead and replace spot resistance welding electrodes.
.3 It should not shoo-t harm~ul fumes.
C. }'~IIJTlN~ RI~QUl~M~NTS
a. 1 The plating should not cau.~e indesirable phosphatation
~3rflects .
C.2 It should show good conductivity and adhe~iveness to
electrophoresis.
C.3 It should not bring about hydrogen craters, when the
electrophoresis is ol the cataphoretic type.,
C,4 It should ~uarantee a proper paint adhesion7 both im-
mediately a~'ter application, and after thé start of
corro~ion proce3ses.
C.5 It should not bring about corro3ion products that 9 on
account o~` volwne increase, may cause the pairt to swell
and chip off.
C.6 No sur~ace roughnes3, noticeable below the pairlt.
D. UTILIZA'rl0N_~'f~,UI~ .qf~`NTS
D.1 High resi~.3tance to all, kin,ls of corrosion or in~uced
microclimate, whether acid 9 alkaline or saline.
-- 14 --
~L2367~L
Al'chou~rh thi~ in~ Ltio~ rerers to ;~hatsoever type of inor-
gan:ic cleeiIolytic ~eposit obtainable on a previously gal-
~anized surfcce, it i5 pclrti.clAlarly concerned vYith -the ease
Y/herelrl tihe t'irst ~leetrolytie ].clyer follo~,ving galvaniz-ltion
hac; a r.letal chrornium blse, and the ~.econd hll~ a trlv~lent
chrfj~iur.l b~sfe that, t~lrou~h ehemiccLl rc1etion qvith the ele~
troly~is b.lth, is -trans~orr,e~ in-to chrolrlium h~1roxide and
a~terl~Jclrds" through dehy~ration, into ehrolrium oxide CrOx.
hll thf te~sts rllade have 3ho~qn that this tyi~,e ol rnulti-layer
eleetroLy-tie treatlnent r.reet3 bes-t -Lll the ~bove listecl eh~
raeteristics (~,oints A to D).
Moreover, the fol1Gwinc~r ~rocess suit-Lbility requir-err.ents
must be taKen into aeco~rlt in or~ier to obtain an optimal
product ~'or the r~.o-torvehiele ~ nufLeturing industry.
. f~ANUF~v'l'lJRlNG PROCESS ~Ef~UI?~2-1~F`~ITS
1~'.1 Corrp.-let aiditlon.-Ll electrolytie -treatMer~t units, to be
ir)~erte~ in the final pclrt o~ any zine-based plcLting
proces~.
~`.2 F.L3Y trf~Ltf!ien-t either on one or both ~idf.~s,.
E.3 Reproducibility and stea~ra~-tne~ss, -t~ieal of eleetro
ly-tie ~roeesse~. '
36~
.4 Comr)~ct pro~uctivity, a],so upstrea~ of ~ ~actory blan~;
cutting line, or upstrealr! of a pre-painting install~tion
or Or whatsoever g~lvani~ed steel ~inishing installa-
tion.
I'l,IN~ ' TEIE ~ROCl~'SS ~CC~KDING TO TH'/` I~V~N'L'I~N
There are two ways to carry out on galvanized steels a c~lro
mium and trivalent chromium electrodeposition process, react
ing to form a l~1yer of hydroxide, and thereafter of' oxide.
~arnely:
- in -two successive stages, by deposlting first the metal
chromiurn anl therl the triv~lent chrornium~ so as to generate
the oxide la,yer, by means of separate electrolysis tanks
for each depo~ition process.
- in a sin~rle stage, by depo3i-ting first -the chromium, in
the ~inal stage of the process the trivalent chromium will
be caused to deposit and -transform itselr into chrornium
oxide.
Ex~erience has shown that the two-stage process is the best
to industrial applications.
There are four wclys by which -the process according -to the
inven-tion can be carried out:
_ 16 --
~ 36~9~l
- in the fin~l area o~ a galvanization line;
- in a separate, self-su~ficien-t installation;
- at the head of a galvanized steel finishing installatiotl,
such as, ~or instance, the cut-ting dept. of rolls into
8hQet~Y; ~'
- at the head oi' a pre-pain-ting or plastic film application
installation.
In each of the above ca~es, the s-teel surfaces, whether gal
vanized or no-t, ~nust be suitably de-greased and cleaned be
~ore they reach the chromium - trivalent chromiurn electro-
lytic treatr~ent UllitS.
The above is usually done in the galvanizing lines by degreas
ing (e~g. with trichloroethylene) and~or electro].ytic pi--
ckling, and/or che~lcal pickling, and/or electrolytic pi-
ckling in neutlal salts, and/or al}caline washing ~nd final
cle~-Lnsing Nith ~ater, hot if possi.ble.
The technical aspects o;t' -this stage of the process being
known, it will not be dealt ~ith in detail; it is therefore
taken for gr~nte(l that the galvanized steel is clean when
it, reaclLes the electrolytic install.ltion speclfi.ed in thc
present invention.
~3~7~
'~WO-S'I'A~E PROC~SS
1~ ~lectrol,~Jtic deposition o~ r.~etal chror~,ium
'llhe electrol~tic deposition of chroriium on a galv~nized
~urface, in order to ~rive it ol~timal corrosion resistance
qualities, c~n be ~one by means of a ~irst-rate combi.natlon,
at least as far as the f'ollo~ing p~rametels are concerned:
- electrolytic bath composition
electrolytic ba-th temperature
- types of anodes, and their arrangements
- c~thodic current density.
or each of' the above ,undamental parameters, the specifi-
cation will indicate -the large value ranges wherein the pro
cess may be carried out, as well as the values which, base~
on experie1lce, have proven to be optirmal.
~lectro~ c b~lth co,~lpox1tio_
The metal chromium deposition ba-th onto the galvanized sur
raOe consists o~ two fundar..ental components: ch.romic anhy-
dride (CrO3) in its quality ~s supI)lier oi~ ion~ Cr~6, and
sulphuric acid, whose ions SO~ ac-t a--l catalist~ in the
elec-trodeposition proce~3.
'l'he ,sulpnuric acid can be integrated and substi-t~lted b~ r-
~ 3~9~lsulphate. ~or high deposition speeds, the bath must be inte
grated by other catalysts.
Ch.rorl.lc anh~drid ~ 3) contenr,s in a~ueous solution
possible r~nge f 50g/liter to 145g/li-ter
optil~urrl range: 100g/li-ter to 130g/liter.
Note -that Nhen the CrO3 contents are more than 130~/liter,
the hexav~lent chromiurn content o~ the f'umes caused by the
proccss may constitute a dan~er of environment pollution in
the vicinity of the treatment installation~
There~ore, the concentration should not exceed the above va
lue, It ~houl~ be note~, as a point o~ rel`erence, that the
maxi~um chromic anhydri.de/air cubic r,leter concentrati.on a~l
mitted by -the American Conference o-~ Governmental Industrial
Hygierlists for an ~-hour continuous exposure 9 ig 0. 1 mg.
Su~huric~ ac~i~ con-tentC~ (weight r~tio CLO3: ~0~~):
~ossible range: 25:1 to 250:1
optinluln range: 90:1 to 110:1
Notc that ~or ratios under 50:1 the process current ef~i-
ciency is reducel appro~ir~.ately by 15~, and that ~or ratios
over 150:1 the said e~-flciency is even more drastically rr~
duced,
As previously sai.d, the ~ulphuric acid can be int~ratel
- 19 -
~23~7~
or sllbsJGitJuted by a sulphate 9 such rlS ~ Xor instance, stron
t-iur.~ su].phate.
Conten-t~, o.~ other cata]~Jzing~ ents
The chroi!~um der;ositi.on. ba-th current ePficiency is quite
~ood when t'he above sald CrO3 and sul~ ate ion S04 contents
are L~resent.
rt i.s pos;ible, however~ to obtain a hig~er current e~ficien
cy by adding other catalysts and optimizers of the bath
elec-trolytic con~uctibility.
~he range of such chror~ium plating bath variations can be
so Nide as to ~n.~ke it imposslble to cover all o~ them.
'l'hey are, iIl any case~ ~ui.te essential to high treatment
speed installati.ons; thi.s is a list oi' somo o-~ the ~ossibl.e
trea-tments: addi-tion o-~` hydroYluoric acid and/or Yluor~.des,
and/or rluOSilicic acid; and/or ~luosilicates, an~or cr~o
lite, and/or f'luoboric acidj and/or Yluoborates, and/or bo
ric ~lci~l.
~dditi.on of l~ , and/or SiF ~, an~/or A~' 3 an ~ or BO 3
_ _ 6-- ~ - 3 -
i.on a:lder~
_
~,os~,ible range: 0.15 g/li-ter to 15 g/li-ter
ol~tirnurl~ range: 1.2 g/liter to 1.7 g/llter
Aidition Or_~F4_ ion ad~er3 (40-~,~ solution~
20 -
~2 ~ 9 ~
possible range: 0.2 ml/liter to 5 Inl/liter
optlmum ranre: 0.4 ml/llter - 0.6 ml/litel
Note -that the above Lluoride bas~d catalysts are necessary
to increa~e the current e~ iciency, when, due to the high
~l~ocd oI' the gal~nizéd steel band vvhich must be plated
wlth chrorniur.~, (over 20 - ~0 m per m,inute) there ir, not en-
ough ~pace to obtain an adequate plating thickness. In ~ar
-tioular, the above optirrium values relate to a plant whereln
the band feed is 30-40 meters per minute wlth 6 to 8 ~m
thickness o~ the zinc to be alded.
A~ ~ill be seen herein~fter, it will be necessary to resort
to special lead alloy ano~es, in or~er to contain the l'l.~or-
ide attack.
Bath contamination contro].
. _ . . .
Due to its peculiar proce~s, the chromic acid bath gets in
oont;lct ;lith materials which Iray pass on to it some foreign
mclt-ter: anodes, the ~inc pla-ted band ~ide, the bare steel
;~ide (if it i3 a "one-side" product).
~oreover the bath, on account o~ -the electro.lepor,ition pro
oess i~hich it must carry out, may cause the hexavalent chro
mium to be reduced to trivalent chromium.
Beyond giverl values, the pre~ence o~ contariin~ltin~ elerr,ents
~ 21 -
~3~9~
may cut doNn the process current efficiency.
It is a good principle that the iron, co~!per and zinc con-
tents of the bath sllould not .ss a i~hole exceed a 10 g/liter
value. To contI ol such con.tents, it i.s advisable to provide
for the possibility of? re-circulating the electro].ytic so-
lution by mcans oY ~uitable ion-exchan~er resins 5 not con-
tinuously, but from time to time when the solution begins
to be contarllinated.
Based on our experierlce, the treatr;ent ought to be mad.e eve
ry 500 tons of' steel produced.
As ~or trivalent chromium, its presence should not be in
excess of 1.5 - 10 gfliter, so as to preven-t current ef~i-
ciency reductions.
Anocles. Contrary to other electrolytic proces3es insoluble
anodes ara used in -this case. It is also possible to mlake
use of conventional anodes consisting oY copper bclrs plated
with lea~, tin-lead, antirf,ony-lead, anti~.ony-tin-l,ead 9 tin-
~.ilver-lead.
The electrodes can also be ~.~holly r.~ade of leld or lead-alloy.s.
1 t iS iraportant to balance the rr.a3s ~,nd surY~ce oY the a.no
des ~ith the current density, in orde r to avoid te~.perature
increases, particul~rly ,~hen using ca-talysts ~)erlnit'ing to
~ 2367~
operate at a high current density.
It is al30 possible to resor-t to rnild steel anodes, in order
to check the trea-tment cost. In this c~se, it is necessary
to kceL) under closer control the iron ions contents o~ the
solution~ hence the current efficiency reduction.
It is also pos~ible to use graphite or titanium anode3 - or
all oy9 thereoP.
Ar~n~ment Or anc)des. Ano1es can take ~ha~30ever angular
position - lrom perpendicular to almo~t parallel - vith re
spect to the band feed direction,
ln our ex~ericnce, the best arrangerl~nt is the one ~herein
the anodes ~orm an 8-9 angle -~ith the band feed direction.
It is important to co~bine the angular position, lenght and
width o~ the anodes so 1nat the whole band wid~n il,ay ,e}nair
for an equ~l time under the surface covered by e]ectrodes.
~he ~nodes' ~eometrlc arrange.~lent, independently ot' their
an~ular position vit;h respect to the band feecl axis, can be
hori~ontal, vertical, or radial (ce~ll and anodes' geometry).
B te~l~ercLtur
Our e~perience ha3 shown that ~he optimum hath -telllperature,
in -the current density rar.ge adopted, is 45 to 65C.The l~ol
lowing -table 3hows 9 by approY~imcltic)n, the current densities
-- 23
1233~7
and the corresponding opti~um bath temperatures.
~empera-ture Current den~
-
~5 - 4~C 10-30 A/d.~l2
~8 - 550~ , 30_45 A/~2
55 65C 45-90 A/drn2
It i~ advis~able to identify 'by means o~ a Hul]. cell (mea-
suring the current efficiency) the op-timum tempera-ture in
a 30 to 80C field, within the above lis~ted current range~s
in ~unction of the bath cor.~position, o~ the electrodeposi-
tion cell geometry and o~ the electric fie]d. ln general,
i~ only ions S04 are present as catalysts, it is preferable
to stick to the lowest temperature range values; i~ other
catalysts, such as f'luorides, are present, it is pre~erable
to operate at the highest teml,erature range. In all cases,
however, heat exchange shculd be provided f.`or, so as to sub
tract -the heat developed hy the curxent passacge~ and keep
the solution temperature at the pre-establi3hed value (ideal
ly within + 2~C).
Catholic cur_cnt ;len3it,y
The ourren-t density required ror chrollium electrodeposition
on galvallized steel ranges fro~ 15 to 15~ ~/dm2. The optimal
- 24 -
~236~
values iar e bet~veen 50 ~nd 75 A/dm2.
Wei~ht of del,osited chromium ~)er surP~ce unit
_. .. . . . _
~he invention relates to tne electrodeposi,tion of chromium
we ighin~ up to 5 g/m .
The optimal chrornium weight being~ a f~ir balancs between
co~t, tre~tm~nt speed and corro.sicn resis-tance, ranges Prom
0.55 to 1.~5 g/m .
'~he said weigh-t ranges, which are translatable into platin~
thicknesses, can be obtained by operating in the optimal con
ditions of the various above 3aid parameters, in an electro
deposition area (covered by the anodes) of approxirnately
1 m every 20 ~/minute band ~eed speed.
That is to say, if the banl Peed speed is equ~l to 60 rn/r
nute, ~he lenght o~ the chromlum electro~eposition area
shall be approxlrnately 3 rlleters~ .
'l'he sai~ leng~h-ts o-~ -the effective elec-trodeposltion areas
are onl.y approxi~.ate being iniluence~ by the f`ol:lowing: b~th
composi-tion, cel.L & ~no~le geo~ietry, current e:~fici.ency as
well as by -the way the solutiorl lS f'ed ln-to the depoc,itior
c~rea. To -thls end, i-t is advisable Por. the solution -to re-
circulclte counter current to the steel bancl feed.
A w~shing oper~tion should be provided ~or at the end o: the
-- 25 --
31236~9
~irst sta~e 9 ~ossibly with hot ~later, to prevent the second
staga bath contamination, particularly with SO~ ions.
2. Ie~osition_of trivalent chromium wh] h, b~
rt.,~lctin~ i.t~l the b~th ~ L~L~ ~9=
~-,f r~ e h~r,ltion, chromiurl~ oxi(le.
The pu-rpo~e o~ the electrolytic deposition o:~ Sl trivalent
chroJniurn catholic filrfi on top of the electrolytic chromium
layar deposited during the ~irst galvanizel steel treatrr.ent
stage, is to obtain - through chemical reactions with the
bath - the ~ormatiGn of chrorniurri hydroxi~e which in turn~
becoming dehydrate~, lead3 to the f`orrnati.on of chromium ox
ide CrOx~
The ~unc-tion o~ the chror,~ium oxide layer is to seal, to ~as
sivate the chromiurrl an~ successively to ~'ix the pain-ting
treatincnts; it is important to this end, tha-t chromium com
pounds, with valence 3 or less, should be pre~ent.
'rhe electrolytic dc,posi-tion o~' the trivalent chrorium ca-
thod:i.c film and the successi.ve reactiGns ¢an be -thus clari
fried .
~y Irleans of ~lst cathodi.c scan~ion~ of galvanized steels,
it W~lS possib].e to observe the ~lotentio-~ynamic curves o~
~ 26 ~
~.~3~
chromic anhydride solutions, ~ well ~s -the following suc-
cessions of cathodic reactions.
At the less negative potentials (-200 . - 600 ~V) hexava-
lent chro~nium is reduced to trivalent chro~lium.
Around - 800 rnV there is a first hydrogen forrnation which
tends to incre~se the pH in pro~imity to the electrode; it
i~ therefore likely that the hydro~ide Cr(OH)x is forrned du
rin~ this stage.
~or the metal chromiuln to deposit, more negati-re potentials
mu~t be attained (-1~00 mV). Such elec-trodic reac-tion takes
p]ace almost at the same ~otential~ as with a hi~h hydrogen
ion reduction; therefore, there is a re.~arkable evolution
of g~seous hydrogen.
Consequently, in the case o~ rnetal chroniium deposition, the
ap~lièd current ~ensities, which YiY~ the potential ? fr.ust be
higher than in the case of deposition of the -trlva:Lent chro
m:luln catho~ic fil~n. '~here is hydro~en evc].ution in both Cl
se~ uch evolu-tion lS luite con3idera~1e in the case o~ ~i.e
tal chrornium deposition, and it is unde3:irable a3 it re~uces
the curlent e~ficicnc~ for proccsses purposes (chromiulll de
position); on the o-ther hanl, ln the deposition o-~ trivalent
chromiuDI it ie-J essential Lor a hydro~en evoluti.on to tak~
27
~2~679~L
place, e~-en lf' less consi~erably than in the previous case,
to obtain the procass ,vanted, namely the cheL~ical reaction
leading to the formation of chromiun; oxide.
Chromium o};ide i~ ~'ormed rrO~ hydroxide, followin~ natural
or Lorce-l dehydration.
The electrolytic deposition of the trivalent chromium film
~nd its succe~sive chemical transformation into chromiu~
hydroxide is obtainable through an optimal combination ot'
at least the follo~iving:
- electrolytic bath co~position
- electrolytic bath temperature
- types of anodes
- c~thodic current density
Wide value ran~es within Nhich the process can be carried
out "vill be given for each of' the above parlrleter~, wi-th
the inlication cr those value ,vhich, in our e~per:lence;
have I~roven to be optil~al.
lectrol,xtic bath comr~osition
The trivalent chromiuln ~lepositiorl bath on-to the r,etal chro
Mi.um sur~'ace consist~ o two fundamental comporlents: chro-
mic anh~dride (CrO3) as supplier of ions Cr+6, ~-nd a ba~e,
such as ~odiu~!l h~droxi;~e (NaO~) as pH regula-tor.
- 28 ~
~367~
ChrolLic~3~ (CrO3 ~ solut on
possible range: 10 g/liter to 49 g/liter
optimal ran~e: ~5 g/liter to 45 g/liter
~aOH content: ~o~llum hydroxide , or any other ba~e, ~hall
be added only as tnodifier of the pTI, ~hich .~IUSt as~ume the
rollo~inL~, va].ue~:
possible ran~e: pH greater than 2
optirr.al range: pH from 3 to 5
Should the pH be ~rom O to 3, there is the danger of chemi
cal passivation ~ue to surface chror.atation. 'l~he cher~ical
chrorrlat~tion salts contain highly toxic he~avalcnt chro~iu~,
which is unsuita~le to motorveh-cles.
Also in the case of trivalent chrolrlium electrodeposition it
is possibLe to add tG the bath some catalysts and activa-tors
of the so.Lution conductibilit~, as in the previollsly men-
tioned ca~e of metal chromium depo~ition. It is ~)referable
not to resort to sulpnuric acid or sulphates 9 in vie~/ of the
:L~act that they are speclric catalys-ts o~ the met;al chrorrlium
e~lectro~eposition, an~ not o~ that o~ trivalerlt chroMium.
~ath ter~_r~-l-ture
, . . . . ...
The trivalent chromi~ eposltion is fo~-tered by lo~er -tem
peratures th~-~n those needed lor the deposi-tion of metal
-- 29 --
~ ;23679
chrorniu~:
po~ible r~ng~re: 10 to 45~C
optimal range: 20 to 25C
Th~reI~ore~ the heat ~enerated by the current ~a3sage mu~t
be t~kcn O~r the co~po~ition, by means of an exchangsr.
Anode~. The t~ier~ o~ anocle~ and their ~ngullr ~o~ition in
re~pect oI the band feed axis are ~ ilar to those speci-
fied for chrornium ~epo~ition. The same is true eor cell
geometry ~nd anode di~tribution (horizontal, vertical or
radial).
Current d~n~
_ __
pos~ible range: 1 to 21 A/d~2
optim~l range: 10 to 18 A/drn2
Weight of le~osited chromiu.rQ ox ~ f~ce unit
~hc trivllent chrorrliurr. cathodic -rilm, daposited on the pre
~iou~ly applied :rir3-t lsyer of rnetal chromium, reacts with
-the inte:rface ~olution, enrichecl with OH ion~ for th~ di3
chflrge Oe hydrogen, thuc~ producing chrol~iurn hydroxide throu~h
chemica.l reaction.
Chromiwll hy~ro~ide, havin~ beil1g ~shed with hot water jet~
nd driecl with hot air jet~, tend~ to dehydra-te and tran~-
.form it~elf in-to ^hromium oxide (CrOx).
-- 30 --
~23~ii7
The weight of chromium oxile deposited ~er area unit is
ca].cul.lted on the ba~is of it3 chromiu~. content.
The l)resent invention rel:tte~ to the follo~ving chromiu~
wei~ht ranges as chro.~ium oxi.de:
po~ible ran~e: up to 1 g/m
IJ't~ l rclnge: 0.035 ~ 0.085 g/m
~he resultin~ surface unit weight ra-tio between the metal
chromium and the chro.ni.um pre~ent in the oxide, may range
from 150:1 u~, to 0.15:1, however, its o~,ti.~-,al Yalues~ o~
econornic and indust~ial interes-t ran~e fro~n 25:1 to 4:1.
These Cr : Cr (in CrOx) ratios are tho~e which turned out
to be optimal in the product tests, in ter~.s of moldi.ng,
~velclin~, painting and corrosion resistance.
A.~, previously said, after the ~econd elec-trolytic treatmen-t
c~3tage, the Ir,ulti.-l~yer plated ~te,el banl is ~ashed possibly
with hot v1ater, and then dried ~1ith hot ai.r jet~3.
Moreover, ~ pasC~age i.nto a 100 ~ 300C ~tove can be provl--
ded ~or to ~acilitate the dehydration of hy~rox.ide.
In our experience, anyho~v, dehydration can al,so take place
in a natural way, and there are no characteristic dif~eren
ce~ ~etween produts dehylrated naturally OI' in a f'urnace.
The ~urther I10ssible treatment~ of' galvanized rrlultilayer
- 31 -
6~ 9 ~
electrolytic steel Cr-CrOx (oiling or phos~,nating or other
equivalent treatment3) are ~uite ~ell kno~vn; therefore, even
when rrentioned, it is obviou~ that they are not a part of
thi~s invention.
A speclQl treatr;ent, based on our experience in the case of
~teels with raultilayer one side ~Ire-plating~ at the end Or
electrochemical proce~se~ l~here the solutions may contarl~irla
te the unplated ~1~e, consists in mechanlcally brushing the
latter.
All preparation, galvanizing and cleansing operations prior
to the Cr ~nd Cr~x electrolytic treat~.ent are not part o-f
this invention.
ONE-STAGE PROC..~'SS
._
Beside~ being in t~o separate stages, one for met~l chro-
mium and one ~or trivalent chromium transrorJned into chro~
mium o.~.cide, the multilayer electrolytic plating of the gal
vanized steel surf~ces can also consist o~ a ~.in.gle stage
~h.ere the required depositi.ons take place in ~uccession.
'rhis system, though o~ lesser industrial interest, is des
cribed in or~3er to provide ~or all possible ways tc obtain
a multilayer electrolytic pllting, the object Or this in~
ventlon.
~2 -
~ ~ ~ 6~
As in previous c&ses, the one-stage process is chlracte-
rized ~ the ~ollowing par~meters:
- electrolyti.c bath composition
- ~lectrolytic ba-th tem~erature
- type~ of anodes
~ c~thodic current densit~
'l'he large value ranges within which the process can be ca_
ried out and the values con~idere~ optim~l on the basi~ of
ou.r experience will be given for each of the above pa:~ameters.
E.lectrolytic ba-th composi.-ti.on
Chromiu~ ~nh,~dride cont. nt (CrO )
3-
possible range: 20 g/liter to 140 g/lite
~ptimal range: 30 g/liter - 50 g/li-ter
~ul~huric acid contents (weight ra-tio CrO : SO
_ _ 3 _4
pGssible range: 25:1 to 250:1
opti.mal range: ~0:1 -to 100:1
Also in -the slngle-stage procc~ lt i~ possible to resort
to cat~lysts incre.~sing the bath current ef~`iciency, as in
the two-~ta~e proces~.
Con-tent o~ compounds suppliers ol' lons ~ ; S~ , Al~6 3,
03 3
pos~ible ran~e: 0.15 g/liter to 15 ~/li.ter
~L~3~
optirm,~l ran~e: 1 g/liter to 1.5 g/liter
Electrol tic blth _tem~er,~tur~
possi~le :range: 20 -to 70C
opti~nal range: 30 to 40C
C<.L_hO~ iC CU rrent dtn3it~
~os;3ible range: 10 to 200 A/dm2
or)t,irnal range: 30 to 50 A/dm2
With reg~rd to bath cont~rnination control, ~nodes, ~rran~;-
ment of anode~ reight of chromiurn de~osited per sur1~ace
unit; ~,eigh-t of' chror.rIium oxide, ratio bet.~veen the t~o weigh~s,
see the t/~o-stage process s~ecific~tions.
OP~ATII~IG Tl~'SrS
The oper~ting te3t~ were c,lr~ied ov.t ~i-th the follo.~ing
standard, one si~e ~)latecl, product:
Steel: ~'e P04
Si%e: 1500 x 0.~ rnm
Zinc plat:ing thickness: ~.3~n~ (electrolytlc ~alvanization
~roces,s)
'I'hlcknes3 of chromium plating: o,~,4 g/m2
~hic~ne8s OL chro;,~ium oxide r~lating: 0.041 g~m'-
Moldin~
- the n]ultil,~yer Zn Cr~CrOx platin~ doe.s not modi~y t~e b~-
-- 34 --
~:36~9
sic steel mol.~ability~- the rnultilayer ~lating does not flake up to the lirnit steel
molding curve
- the zinc plati.ng does not crack in ~efor,rlation3 induced
~y dr~wing, Nhereas it ~hows microcracking in more severe
defOrlrlatiOn ~llC to s-tretching, Of' the like.
- the p].ating does not turn out to be craclred when block
lapped.
~eld~
- The mechanical and dir.rlensional characteristics of the wel
ding spots remain acceptable up to 10,000 consecutive spots,
the e1ectrode being opposite to the plated sur~ace.
-- Up to 10,000 spots wi.th a stati.onary .~/elder, and up -to
2,000 spots with a mobile welder, there is no need to bead
the electrodes.
- No zinc or chromium arc detected in the analysi3 o:E fulT.es
rrom 100 ~eldin~ spots.
a :Lnt :i~
No hydrogen crater3 are pre3ent after primer annealing up
to the ~pplic~tion oY 400 V (negative) in cataphoresis.
- ~he paint adhesion~ te3ted after T folding and cathodic
de-l.arn.ina-tion, i3 cornplete~
- ~5 -
~23679
- the catha~oretic primer thickness is greater than the one
obtainable on a phosphatized bare steel sheet, the electro
deposition voltage being equal.
Corrosion test
- When unpainted, the Lr,ultilayer plating begins tb show so~e
red corrosion in a salty rnis-t chamber a~ter 800 hours a i.e.
it turns out to be 10 times as resistant as the conventional
galv~nized products with the sare zinc thic~ness.
- When painted with cathaphoretic primer, it cornes unda~a-
ged out of the scab corrosion test, made according to Volvo
Std 1027
- Again when pàinted, a cut-out cathaphoretic primer does
not show any white or red corrosion in the vicin:ity o~ the
cut-ting a~ter 750 hours in a salty ~liSt chamber
- I-t continues to protect areas with welding spots ~or over
750 hours in a salty mist char~ber.
- When mounted on a .~otorvehicle it ~asses a double Arizona
~est, wi-thout showin~ any signs of corrosion.
- It does not cause ~ny galvanic corrosion, i~ it is joine~
with a bare stee~ sheet section, and painted in anaphore3is.
